Laser processing chamber with cassette cell

ABSTRACT

A process chamber for carrying out laser treatments, on the surface of an object, comprising: a base provided with object support means; a cover provided with a window substantially transmissive of laser light; gas inlet and gas outlet means; the said cover and the said base, when connected, leaving a space between the surface of the element and the inner surface of the window, in which gases flowing through the said gas inlet may flow above the surface of the object being treated and out of the process chamber through the said gas outlet.

FIELD OF THE INVENTION

The present invention relates to U.V. laser surface treatment methods,particularly to the removal of foreign materials from substratesurfaces. More particularly, the invention relates to a process chamberfor the aforesaid purposes, which provides effective dry laser strippingand cleaning.

BACKGROUND OF THE INVENTION

In the manufacturing of various products it is necessary to apply alayer of protective material on a surface, which must be removed after aspecified manufacturing step has been concluded. An example of suchprocess is the so-called "masking", where a pattern is created on asurface using a layer of protective material illuminated through a mask,and the surface is then treated with a developer which removes materialfrom the unmasked portions of the surface, therefore leaving apredetermined pattern. The surface is then treated by ion implantationor by etching agents, which introduce the implanted species into theunmasked portions of the surface, or removes material from unmaskedportions. Once these processes are completed, the role of the protectingmask ends and it must be removed. The process is conventional and wellknown in the art, and is described, e.g., in U.S. Pat. No. 5,114,834.

Two main photoresist stripping methods exist in the modern VLSI/ULSI(Very/Ultra Large Scale Integration) circuits industry:

1) Wet stripping which uses acids or organic solvents;

2) Dry stripping, which uses plasma, O₃, O₃ /N₂ O or U.V./O₃ -basedstripping.

Both methods are problematic and far from being complete, especiallywhen taking into consideration the future miniaturization in theVLSI/ULSI industry. The current technology is capable of dealing withdevices having feature sizes of about 0.5 μm, but before the end of thecentury the expectation is that the workable size of the devices isexpected to be reduced to 0.25 μm. The expected size changes requireconsiderable changes in the manufacturing technology, particularly inthe stripping stage. The prior art photoresist stripping techniquesdescribed above will be unsuitable for future devices, as explainedhereinafter.

Utilizing only the wet stripping method is not a perfect solution, as itcannot completely strip photoresist after tough processes that changethe chemical and physical properties of the photoresist in a way that itmakes its removal very difficult. Such processes include, e.g.,. HighDose Implantation (HDI), reactive Ion Etching (RIE), deep U.V. curingand high temperatures post-bake. After HDI or RIE the side walls of theimplanted patterns or of the etched walls are the most difficult toremove.

In addition, the wet method has some other problems: the strength ofstripping solution changes with time, the accumulated contamination insolution can be a source of particles which adversely affect theperformance of the wafer, the corrosive and toxic content of strippingchemicals imposes high handling and disposal costs, and liquid phasesurface tension and mass transport tend to make photoresist removaluneven and difficult.

The dry method also suffers from some major drawbacks, especially frommetallic and particulate contamination, damage due to plasma: charges,currents, electric fields and plasma-induced U.V. radiation, as well astemperature-induced damage, and, especially, incomplete removal. Duringvarious fabrication stages, as discussed above, the photoresist suffersfrom chemical and physical changes which harden it, and this makes thestripping processes of the prior art extremely difficult to carry out.Usually a plurality of sequential steps, involving wet and dry processesare needed to remove completely the photoresist.

The art has addressed this problem in many ways, and commercialphotoresist dry removal apparatus is available, which uses differenttechnologies. For instance, UV ashers are sold, e.g. by Hitachi, Japan(UA-3150A), dry chemical ashers are also available, e.g., by FusionSemiconductor Systems, U.S.A., which utilize nitrous oxide and ozone toremove the photoresist by chemical ashing, microwave plasma ashing isalso effected, e.g., as in the UNA-200 Asher (ULVAC Japan Ltd.). Alsoplasma photoresist removal is employed and is commercially available,e.g., as in the Aspen apparatus (Mattson Technology, U.S.A.), and in theAURA 200 (GASONICS IPC, U.S.A.).

More recently, photoresist removal has been achieved by ablation, usinglaser UV radiation, in an oxidizing environment, as described in U.S.Pat. No. 5,114,834. The ablation process is caused by strong absorptionof the laser pulse energy by the photoresist. The process is a localizedejection of the photoresist layer to the ambient gas, associated with ablast wave due to chemical bonds breaking in the photoresist and instantheating. The partly gasified and partly fragmented photoresist is blownupwards away from the surface, and instantly heats the ambient gas. Fastcombustion of the ablation products occurs, due to the blast wave andmay also be due to the photochemical reaction of the UV laser radiationand the process gases. The main essence of the process is laser ablationwith combustion of the ablated photoresist which occurs in a reactivegas flowing through an irradiation zone. The combination of laserradiation and fast combustion provides instantaneous lowering of theablation threshold of hard parts of the photoresist (side walls). Thecombusted ablation products are then removed by vacuum suction, or bygas sweeping leaving a completely clean surface.

The aforementioned patent U.S. Pat. No. 5,114,834 does not describe anyparticular requirements for the ablation cell, which is referred to as a"container" or a "process chamber". However, it has been found that thestructure of the process chamber has a critical effect on the quality ofthe stripping process.

While reference will be made throughout this specification to theablation of photoresist from semiconductor wafers, this will be done forthe sake of simplicity, and because it represents a well known andwidely approached problem. It should be understood, however, that theinvention described hereinafter is by no means limited to the strippingof photoresist from wafers, but it applies, mutatis mutandis, to manyother applications, such as stripping and cleaning of photoresist fromFlat Panel Displays (FPD) or removal of residues from different objects,such as lenses, semiconductor wafers, or photo-masks.

SUMMARY OF THE INVENTION

It has now been found, and this is an object of the invention, that notevery dimension and configuration of the process chamber for carryingout pulsed U.V.-laser ablation/etching of foreign materials fromsubstrate surface in ambient reactive gases, provides goods results andthat in order to obtain satisfactory results there are certaindimensional constraints to be observed. These constraints are importantas they provide conditions of reactive gas decomposition and excitationfor stripping and fast enhanced combustion of the ablation products.

The invention provides a stripping apparatus employing a process chamberwhere the laser ablation/etching takes place in a cassette cellconfiguration in ambient reactive gases, which can be used for theeffective stripping or removal of coatings, such as photoresist, toyield a cleaned product of high quality.

DETAILED DESCRIPTION OF THE INVENTION

The process chamber for carrying out pulsed U.V.-laser ablationprocesses on the surface of an object in ambient reactive gases,according to the invention, comprises:

a base provided with object support means;

a cover provided with a window substantially transmissive of laserlight;

reactive gas inlet and reactive gas outlet means; the said cover and thesaid base, when connected, leaving a space between the surface of theelement and the inner surface of the window, in which gases flowingthrough the said gas inlet may flow above the surface of the objectbeing treated and out of the cell through the said gas outlet.

In order to obtain optimal results, fast ignition and combustion ofablation products are needed. This is needed in order to achieve maximaland complete burning, volatilization and removal of such products duringthe short interval between laser pulses (about 10⁻² s). According to apreferred embodiment of the invention, this is achieved by providing acell in which the product of the total pressure in the cell, (P) and ofthe distance (h) between the surface of the object to be treated and theinner surface of the window (hereinafter referred to as "gap" (h)), isapproximately constant. This means maintaining the same amount ofoxidizer needed for combustion as required by the stoichiometry.Mathematically, it can be expressed as P·h=K. K is constant for a givenset up of ablation parameters as laser fluence energy at a givenwavelength. A typical range of K is 40-60 Pa·m or N·m⁻¹ with an averagevalue of 50 Pa·m.

For a typical working pressure of 250 mbar or 2·5×10⁴ Pa, the gap h canbe calculated as h=k/P=50/2.5×10⁴ =2×10⁻³ m=2 mm.

A variety of construction materials can be employed in the constructionof the process chamber of the invention. According to one preferredembodiment of the invention, the window is made of fused silica, quartz,MgF₂, CaF₂ and sapphire, or the like material. According to anotherpreferred embodiment of the invention, the base and the cover of thecell are made of a material selected from among quartz, stainless steel(e.g., 316 L), and aluminum, preferably "hard-anodized", or from ceramicmaterials, e.g. alumina.

Pressurization of the cell can be obtained by any suitable means knownin the art, e.g., by means of sealing rings, such as O-rings.

The above and other characteristics and advantages of the invention willbe better understood through the following illustrative andnon-limitative description of preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a general side view of a process chamber according to onepreferred embodiment of the invention, some internal functional elementsbeing outlined in broken lines;

FIG. 2A is a cross-sectional view of the chamber of FIG. 1, taken alongthe 2--2 plane and FIGS. 2B and 2C are details of FIG. 2A;

FIG. 3 is an enlarged cross-section of the gas inlet assembly, takenalong the 3--3 plane of FIG. 2;

FIG. 4 is a bottom view of the process chamber of FIG. 1; and

FIG. 5 is a top view of the process chamber of FIG. 1.

DETAILED DESCRIPTION OF THE DRAWINGS

Looking now at FIG. 1, numeral 1 generally indicates a process chamberconsisting of a base 2, and a cover 3, which are connected by air-tightconnections (not shown), so that the inner part of the process chamberdefined by the said base 2 and cover 3 can be kept under pressure orvacuum. The base 2 is provided with N_(x) O_(y) gas inlet 4, and O₂ /O₃inlet (shown in FIG. 3), which will be further described below, and gasoutlet 5, for exhausting gases which have passed through the irradiatedzone. The base 2 is further provided with a chuck 6, on which theelement to be stripped, e.g., a wafer, is supported. Through the centerof chuck 6 vacuum is applied, to hold the wafer in place during theprocess.

Looking now at the parts shown in broken lines, the inlet gas andstagnation cell 7 is seen, for the introduction and mixing of inflowinggases, as well as the exhaust assembly 8. A fused silica window 9 isprovided above the element to be stripped, e.g., a silicon wafer, towhich reference will be made in the following description for the sakeof simplicity. This window permits the passage of the laser beam whichoriginates from a source positioned above the chamber 1. A cover frame10 keeps the silica windows in place, and assists in keeping the chamberpressurized or under vacuum.

FIGS. 2A-2C show in greater details and in cross-section, some of theelements of the process chamber of FIG. 1. Two seals, 11A and 11B areshown in this cross-section, which may be, e.g., O-rings. These twoseals define two vacuum zones in the process chamber:

a) Zone 1, which defines the ablation environment in the irradiationzone. The pressure is maintained by means of throttle valve connected ina closed loop to a pressure controller. Typical pressure is in the rangeof 50-2000 mbar. this pressure regime is defined by seal 11B.

b) Zone 2, which defines the pressure in the outside vacuum channel 20in between seals 11A and 11B.

The pressure in the channel is always much lower than in the processchamber and usually is in the order of a few millibars or typical vacuumobtainable from mechanical vacuum pumps.

The outer vacuum channel 20 has two main purposes:

1. To maintain firmly cover 3 through the aid of the atmosphericpressure.

2. For safety purposes, to avoid the possibility of leaking of hazardousprocess gases through seal 11B. Here, in case of a leak, the gas will besucked by the vacuum pump connected to channel 20.

The wafer is positioned above chuck 6 and below window 9, as indicatedby numeral 12. Wafer 12 can be positioned on chuck 6 in two ways, asshown in FIGS. 2B and 2C. In FIG. 2B the wafer is on top of chuck 6. InFIG. 2C the wafer is immersed inside chuck 6. As stated, the wafer doesnot touch window 9, and there is a distance between them which ispreferably kept in the range of 0.2-10 mm. As explained, this distancecan be varied as long as the product of the values of P×h remainsapproximately constant, wherein P is the pressure above the wafer and his the gap, as hereinbefore defined. The pressure referred to above ismeasured in the center of the process chamber in the irradiation zone.

The space between the window and the wafer defines the ablation cassettecell, through which the gases flow, and in which the ablation productsare jetted from the wafer, ignited and combusted. Looking at gas inletstagnation cell 7, it can be seen that the inflowing gases flow into theablation cell through a communication opening indicated by numeral 13.

The window above the wafer is made of fused silica, because it mustfulfill certain requirements such as optical quality, to permit maximumpassage of the incident laser beam (indicated in the figure by the LBarrow), durability, resistance to process gases and temperature,mechanical strength, etc. However, it is clear that alternativematerials can be used, as long as they meet the desired operationalconditions.

FIG. 3 shows the gas inlet stagnation cell, according to one embodimentof the invention. The stagnation cell is shown in enlarged partialcross-section, taken along the C--C plane of FIG. 2. The stagnation cellcomprises gas inlet 4. consisting of three separate inlets, two inlets14 and 14', for N_(x) O_(y) gas, and one inlet 15, for O₂ /O₃ gas. Thegas generator, as well as the O₃ or N_(x) O_(y) generator, if any, arelocated near the process chamber. As it can be seen from the figures,the gases enter separately into the process chamber and are mixed onlywhile the O₂₊ O₃ gas passes the outlet of the N_(x) O_(y) nozzle 17. TheN_(x) O_(y) is introduced through small holes 18.

FIG. 4 shows the bottom of the process chamber of FIG. 1, from which thebottom of the chuck 6 can be seen, as well as the exhaust 5 and inlets14, 14' and 15 of the gas inlet assembly. It should be noted that twoinlets are provided in this embodiment for N_(x) O_(y), while only oneinlet is provided for O₃. However, different gas inlets can of course beused, as long as the gases are introduced separately, without exceedingthe scope of the invention.

FIG. 5 shows a top view of the process chamber of FIG. 1, from which thewafer 12 can be seen below the window 9. According to the particularembodiment of FIG. 5, the wafer and the cover frame are positionedasymmetrically with respect to the cover. This is done in order topermit a laser cleaning while scanning of the outlet channel 8 wheresome ablation residues may accumulate.

It should be understood that the process chamber of the invention is notlimited to be used in any particular apparatus, and it can be used inablation stripping and cleaning to be hereinafter mentioned as lasertreatment processes in any suitable system. For instance, the processchamber can be kept stationary while the laser beam scans the wafer, orthe laser beam can be fixed and the process chamber can move by means ofa suitable X-Y system. Additionally, the invention is not limited to anyparticular shape or size of process chamber, and can be used to performlaser treatment processes on much larger or much smaller surfaces, canbe of different shapes, and can employ different construction materials.Furthermore, the cell can be utilized for processes employing a varietyof gases, and is by no means limited to the use with the gasesexemplified in the above illustrative and non-limitative description ofpreferred embodiments.

One version of the process chamber can be utilized for special purposes,where the wafer or object to be cleaned or stripped is maintained at anelevated temperature to assist and enhance the chemical process. Typicaltemperatures can be between 75-350° C. The heat source can be outsidethe chamber, and heat may be provided, e.g., by radiation, or it may beinside and heat may be provided by conduction. In some applications itis also possible to introduce the gases at elevated temperature, tomaintain the desired temperature in time. It is also possible to heatthe outlet channel 8 to temperatures between 150-350° C., to assist thecombustion of the accumulating ablation products by reactive processgases.

What is claimed is:
 1. A process chamber for carrying out laserablation/etching as well as combustion and evacuation of foreignmaterials from substrate surfaces, in ambient gas flow of two reactivegas components, which comprises:(a) a base provided with object supportmeans; (b) a cover provided with a window substantially transmissive oflaser light; (c) an irradiation zone; (d) a stagnation chamber; (e)separate inlets into said stagnation chamber for said two components ofthe reactive gas; (f) a reactive gas outlet; (g) an outside vacuumchannel; (h) a first seal defining a first vacuum zone in the processchamber; and (i) a second seal defining with said first seal a secondvacuum zone defining the pressure in said outside vacuum channel.
 2. Achamber according to claim 1, associated with means for maintaining insaid second zone a pressure lower than in said first zone.
 3. A chamberaccording to claim 1, associated with the said outside vacuum channel,for avoiding the damage of leaking the hazardous reactive process gasesto the said outside working environment.
 4. A chamber according to claim1, having structural features and dimensions adapted for laser ablation,etching, cleaning and other laser surface treatments in ambient reactivegas including fast combustion and fast gas flow.
 5. A chamber accordingto claim 1, wherein the product of the pressure (P) and of the distance(h) between the surface of the object to be treated and the innersurface of the window is in the range of 40-60 Pa·m.
 6. A chamberaccording to any one of claims 1 to 3, wherein the distance between thesurface of the object to be treated and the inner surface of the windowis in the range 0.2-10 mm.
 7. A chamber according to claim 1, whereinthe distance between the surface of the object to be treated and theinner surface of the window is about 2 mm.
 8. A chamber according toclaim 1, wherein the window is made of a material chosen from amongsilica quartz, MgF, CaF and sapphire.
 9. A chamber according to claim 1or 2, wherein the base and the cover of the chamber are made of amaterial selected from among quartz, stainless steel, aluminum orceramic materials.
 10. A chamber according to claim 1, whereinpressurization is obtained by means of sealing rings.